The number of contact lens wearers has reached to 16 millions and contact lenses have been widely acknowledged as one of the most popular types of medical equipment. Hydrogel contact lenses take 70% or more of the contact lens market and daily-disposable-typed hydrogel contact lenses have the largest share among hydrogel contact lenses. Generally, daily-disposable-typed hydrogel contact lens wearers can wear contact lenses for 18 hours a day at longest.
Ophthalmic disorders include glaucoma, various infectious diseases and allergies. Particularly, seasonal eye allergies, pollen allergy in particular, are widely observed in daily lives and there is a large demand for easy and effective therapies against the eye allergies. Sodium cromoglycate in the form of eye-drops has been popularly used as a drug for treating eye allergies but, this drug is irritating to eyes when administered so that the drug needs to be used to alleviate the irritating side effect of the drug by adding a refreshing agent, a mucopolysaccharide and the like to the drug (see JP 2004-59583 A and JP 2005-187407 A, the disclosures of which are incorporated herein by reference).
JP 2004-59583 A discloses a technique of alleviating the irritating and foreign-body feelings of a drug containing sodium cromoglycate by compounding the drug with a refreshing agent, while JP 2005-187404 A describes a technique of raising the viscosity of lacrimal fluid by means of an additive of mucopolysaccharide to suppress the absorption of sodium cromoglycate and alleviate the irritation of eyes. However, these techniques cannot satisfactorily alleviate the irritating effect of sodium cromoglycate and thus one or more other drugs need to be added in actual use.
Meanwhile, various techniques of preventing allergies including pollen allergy by using a drug for preventive administration have been discussed. For example, there has been a report that the itchiness due to eye allergies can be suppressed by administering a drug for treating allergy in the form of eye-drops before an occurrence of allergy. However, even if patients who accept suspension of wearing contact lenses and administration of eye-drops after an occurrence of allergy they may, more often than not, be reluctant to suspend wearing contact lenses and to accept administration of eye-drops before an occurrence of allergy. Thus, the preventive therapies of administering a drug for treating seasonal allergy in the form of eye-drops in advance have not become popular.
When contact lens wearers suffer from eye diseases, they need to suspend wearing contact lenses and receive the treatment with use of eye-drops, which is very dissatisfying to them because they have to give up the correction of eyesight during the treatment. For the purpose of dissolving the dissatisfaction, techniques of administering a therapeutic drug by causing contact lenses to contain the therapeutic drug have been disclosed (see JP 2004-307574 A and JP 2003-301014 A, the contents of which are incorporated herein by reference). These techniques are designed to control the release of the therapeutic drug by way of an ion exchange reaction between the components of the contact lens material and the ingredients of the therapeutic drug. JP 2004-307574 A discloses a contact lens for sustained drug release that gradually releases, as the therapeutic drug, an anionic drug having a carboxyl group, a sulfo group and a phosphate group in a molecule, wherein a cationic monomer is used as a component of hydrogel by 2 to 50 mol % and an anionic monomer is compounded with the cationic monomer by 30 to 90 mol %. JP 2003-301014 A discloses a lens for an eye for sustained drug release, wherein the lens is an ionic lens made of a copolymer of a hydrophilic monomer and a methacrylate having a phosphate group in a side chain and also has a cationic substituent in the inside of the polymer.
In the case where a contact lens as described in JP 2004-307574 A or JP 2003-301014 A is used for drug release control, it is characterized in that the ion interactions are strong so that the release rate can be slowed down, which is effective for slowly releasing a drug for a long period of time, at least over 24 hours. However, when the lens is used as daily disposable contact lens that is to be worn for about 18 hours at longest and then discarded, it cannot be expected to provide a remarkable therapeutic effect and hence is not practically feasible.